Since the first incarnation of digital paint systems, there has been a lot of work done to make synthesized paint strokes look like those created using traditional paint and drawing tools, for example, watercolor brushes, chalk strokes, etc. In particular, two known commercial products, "PAINTER" and "EXPRESSION" sold by MetaCreations, provide tools for use in creating "natural media" digital painting and drawing. "PAINTER" is raster based and uses procedural algorithms to generate paint strokes that give the appearance that they have been created by their real world counterpart. "EXPRESSION" is vector based and uses "skeletal strokes" technology.
Skeletal strokes, described in S. C. Hsu and I. H. H. Lee, "Drawing and Animation Using Skeletal Strokes," SIGGRAPH '94 Conference Proceedings, Jul. 1994 and S. C. Hsu, I. H. H. Lee, and N. E. Wiseman, "Skeletal Strokes," UIST '93 Proceedings of the ACM SIGGRAPH and SIGCHI Symposium on User Interface Software Technology, Nov. 1993, utilizes a vector graphics realization of the brush and stroke metaphor using arbitrary pictures as "ink." Generally, defining a skeletal stroke requires drawing an instance of the flesh, which could be any arbitrary picture, around a reference line with a right-angle bend. The reference line provides a reference x-axis for the points specifying the position of the flesh; a reference thickness provides a scale to specify the lateral distance of the these points from the reference line. A picture so anchored to a single reference line defines a skeletal stroke. Once a skeletal stroke is defined, it can be applied along any arbitrary path by simply drawing the path and aligning the reference line of the skeletal stroke with the given path. In this manner, the flesh is distorted (stretched, compressed, bent and/or sheared) to generally follow the path.
While these tools generally work for their intended purpose, both applications tend to fall a bit short in terms of the realism of the paint stroke synthesized. In particular, because of the procedural nature of "PAINTER" and the vector nature of "EXPRESSION", the paint strokes produced lack the subtle flaws and detail of real brush strokes or real objects. Accordingly, a need exists for an apparatus and method for digitally producing brush strokes that appear to be hand drawn and painted.